Field of the Invention
The present invention relates to a radiographic image detector capable of suppressing temperature quenching of the scintillator, which is associated with temperature rise, and also capable of suppressing unevenness in the in-plane brightness of the image.
Description of the Related Art
In recent years, digital radiographic image detectors such as computed radiography (CR) systems and flat panel detectors (FPDs), which allow directly obtaining digital radiographic images and directly displaying images on image displays such as cathode-ray tubes and liquid crystal panels, are widely used for diagnostic imaging in hospitals, clinics, and other places. Recently, a flat panel using a cesium iodide (CsI)-containing scintillator layer in combination with thin film transistors (TFTs) has attracted attention as a high-sensitivity X-ray image visualizing system.
The CsI scintillator has a problem in that the amount of emitted light decreases with increasing temperature. This is considered to be because of an increase in the influence of a phenomenon in which when electron-hole pairs generated in scintillator crystals by X-ray irradiation undergo recombination to emit light, an increase in thermal energy causes electron-hole pairs not to emit light upon recombination but to give thermal energy to the crystals, namely, an increase in the influence of temperature quenching. FPDs, which generally have a heat source such as an electric circuit in the case, also have a problem in that they are potentially more easily heated than CRs without such a heat source.
There is a problem in that such temperature quenching causes a reduction in the amount of the light emitted by the scintillator. In addition, there is another problem in that local variations in the thermal energy applied to the scintillator occur depending on the location and other conditions of the heat source in the case, so that variations occur in the amount of emitted light in the plane of the panel (brightness unevenness). In order to eliminate brightness unevenness, correction by calibration or the like may be used. However, the thermal energy generated from the heat source can randomly change depending on the use conditions, and correction is impossible in some case, which raises a problem.
The problem of the heat in FPD panels has already been discussed, for example, in JP 2014-092447 A and JP 2012-231825 A.
JP 2014-092447 A discloses that a high thermal conductivity case is provided together with a gap provided between the case and a scintillator-semiconductor photodetector unit so that the output from the semiconductor photodetector (corresponding to the TFTs in the present invention) for detecting light generated from the scintillator can be stabilized regardless of the surrounding temperature.
JP 2012-231825 A discloses that an anisotropically thermally conductive plate having thermal conduction anisotropy in a specific direction is provided in a case so that variations in temperature can be made less likely to occur in the detection plane of a detection panel (corresponding to the TFTs in the present invention and the semiconductor photodetector in JP 2014-092447 A).
Both patent documents disclose a member for facilitating heat dissipation or thermal conduction (the case or the anisotropically thermally conductive plate). JP 2014-092447 A discloses that a gap is provided as a measure to suppress sudden thermal conduction or local variations in thermal energy. However, these measures are not based on an idea for suppressing the phenomenon of heat transfer from a heat source such as a circuit board to a scintillator. Therefore, even when these measures are used, an unacceptable reduction in the amount of emitted light from a scintillator or unacceptable variations in brightness can occur depending on the situation where a heat source generates heat.